1. Field of the Invention
The present invention generally relates to active noise control methods and receiver devices which detect and silence ambient noise.
Active noise control methods of silencing ambient noise by outputting a tone having a inversed phase of the absolute value of the ambient noise have been known. Such active noise control methods are applied to air ducts, for instance, to reduce indoor noise, or to handsets to facilitate communication when the ambient noise is high,
2. Description of the Related Art
FIG. 1 shows a conventional active noise control method applied to a duct. A fan or an air conditioner connected to a duct 80 is a noise source 84 generating noise due to the rotation or vibration of the motor or the fan. The noise generated from the noise source 84 is transmitted through the duct 80 and released from an opening. A noise referring microphone 81 is disposed inside the duct 80 near the noise source 84 to detect noise form the noise source 84. The noise referring microphone 81 then transmits a noise signal to a noise processing unit 83.
The noise processing unit 83 turns the noise signal into a noise silencing signal having the same amplitude but an opposite phase by the combination of a filter and an amplifier, for instance. The noise silencing signal is inputted into a speaker 82 to silence the noise transmitted through the duct 80 with the tone having the opposite phase.
A microphone 85 is disposed on the opening side of the duct 80 to detect residual noise after the noise is silenced by the speaker 82, and the noise processing unit 83 is controlled to make the residual noise zero. In such a case, even if a change occurs in the sound transmission characteristics of the duct 80, the residual noise is fed back to the noise processing unit 83 to lower the noise to be released from the opening of the duct 80.
FIG. 2 Illustrates a conventional active noise control method applied to a handset. This figure shows a handset 90, a noise referring microphone 91, a receiver 92, an active noise control circuit 93, a noise processing unit 94, an adder 95, and a microphone 96. The receiver 92 of the handset 90 to be attached to the ear and the noise referring microphone 91 are situated close to each other. The microphone 96 serves as a transmitter which transmits a transmission signal corresponding to the voice. A reception signal from the other end is inputted into the receiver 92 of the handset 90 via the adder 95 to regenerate the voice.
The noise processing unit 94 comprises a filter and an amplifier, like the noise processing unit 83 shown in FIG. 1. The noise referring microphone 91 detects the ambient noise, and the noise processing unit 94 produces a noise silencing signal having the opposite phase from the ambient noise. The noise silencing signal is then inputted into the receiver to output a regenerated voice corresponding to the reception signal and a tone for silencing the ambient noise.
In such a case, the ambient noise is inputted into the noise referring microphone 91 at the same time as it enters the ear. The regenerated voice from the receiver also enters the ear and is inputted into the noise referring microphone 91. Accordingly, the regenerated voice coupled with the receiver 92 and the ambient noise are inputted into the noise referring microphone 91. The noise processing unit 94 needs to produce the noise silencing signal in accordance with the acoustic coupling condition between the receiver 92 and the noise referring microphone 91.
In the first conventional method shown in FIG. 1, to reduce the adverse influence of ambient noise, a microphone is employed to detect the ambient noise outside the handset. The phase of a noise signal from the microphone is inverted. The noise signal is then added to a transmission signal from the handset, and the noise signal and the transmission signal are added to a reception signal from the other end, thereby outputting a tone having the opposite phase from the ambient noise. This type of method is disclosed in Japanese Patent Laid-Open N. 5-110650, for instance.
In the second conventional method shown in FIG. 2, noise is detected in a plurality of positions inside a car or the like, and a tone having the opposite phase from the noise is outputted from a plurality of speakers, so that the residual noise can be minimized. The filter in a noise processing unit is controlled to obtain such an effect. This type of method is disclosed in Japanese Patent Laid-Open No. 6-175668, for instance.
Since no substantial change occurs in the sound characteristics of the duct 80 shown in FIG. 1, the active noise control by the noise processing unit 83 is relatively easy. However, if the active noise control method is applied to the handset 90 shown in FIG. 2, the acoustic space formed between the receiver 92 and the ear greatly changes during communication.
A noise signal from the noise referring microphone 91 derives from ambient noise and acoustic coupling of a regenerated voice from the receiver 92. Accordingly, the noise detection signal greatly changes due to a change of the acoustic space. Even if the noise processing unit 94 has the optimum characteristics, the ambient noise might not be efficiently silenced during communication.
If the noise processing unit 94 includes an amplifier, a loop is formed by the noise processing unit 94, the adder 95, the receiver 92, and the noise referring microphone 91. In such a case, a positive feedback condition might be caused due to the acoustic coupling characteristics between the receiver 92 and the noise referring microphone 91, resulting in reinforcement of the noise.
In the case where the phase of the noise detection signal from a microphone outside the handset is inversed and the inversed noise detection signal is added to a transmission signal and a reception signal, the entire structure cannot cope with a change in the acoustic space between the handset and the ear. As a result, the ambient noise cannot be sufficiently silenced.
In the case where a noise silencing signal to be inputted into the speaker is controlled so that the residual noise can be minimized, the amount of arithmetic operations is extremely large. It is difficult to apply this method to the handset of a telephone, especially to a potable telephone.
A general object of the present invention is to provide an active noise control method and a receiver device in which the above disadvantages are eliminated.
A more specific object of the present invention is to provide an active noise control method in which ambient noise can be silenced in a relatively simple manner.
The above objects of the present invention are achieved by an active noise control method which comprises the steps of: detecting ambient noise by a noise referring microphone; inputting the detected ambient noise into a plurality of noise processing units of different types; producing noise silencing signals by the noise processing units; selecting one of the noise silencing signals; judging whether the selected noise silencing signal exceeds a predetermined range; switching to another one of the noise processing units if the selected noise silencing signal is Nudged to be beyond the predetermined range and thus incompatible; inputting the noise silencing signal into a receiver or a speaker; and silencing the ambient noise with an output tone from the receiver or the speaker.
If the ambient noise cannot be silenced due to a circumstantial change of the ambient noise, switching from one noise silencing signal to another is automatically carried out. Thus, the ambient noise can be sufficiently silenced.
The above objects are also achieved by a receiver device comprising: a noise referring microphone which detects ambient noise and is disposed in the vicinity of a receiver or a speaker to which a reception signal is to be inputted; a plurality of noise processing units of different types into which a noise detection signal from the noise referring microphone is inputted, the noise processing units generating noise silencing signals to silence the ambient noise; a switch control unit which selects one of the noise silencing signals to be inputted into the receiver or speaker together with the reception signal; and a nonconformance detecting unit which judges the selected noise silencing signal to be beyond a predetermined range and thus incompatible, and then instructs the switch control unit to select another one of the noise silencing signals.
The above and other objects and features of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings.